Anti-lock braking systems are known comprising microprocessor controlled devices designed to prevent sudden braking from locking the wheels of an automobile and sending it into a skid. Such devices modulate the brake pressure applied to each wheel individually or to the front wheels individually and the rear wheels together, preferably in response to wheel-speed sensors on each wheel. Such magnetic or electronic sensors constantly monitor individual wheel speed and transmit data up to 2000 times per second to the on-board computer or microprocessor, and thus are capable of detecting rapid changes in wheel rotational speed. Early vacuum-mechanical systems were relatively bulky and costly, and they controlled only the rear wheels, working only to prevent rear-wheel skids. Such early systems did not retain maneuverability during sudden stops, meaning that desirable changes in the vehicle's direction could not be attained.
A typical modern system brakes each wheel appropriately, responding to a lockup signal by automatically pumping the brakes, repeatedly releasing and applying pressure to the brakes as many as twelve cycles per second. Some such modern systems also use hydraulic circuits to provide extra boosts of brake pressure when needed but are very costly. Modern systems also rely on two identical logic circuits which must agree before the anti-lock braking system would function; otherwise, regular driver-controlled braking will remain in effect.
Conventional systems utilize modulators (such as solenoid valves with bypass valves) located at each wheel, or at each front wheel and at the rear-axle pinion to modulate each front wheel separately and the rear wheels together. It is desirable to locate all modulators at one location greatly reducing assembly costs.
It is desirable to gang together all electrical wires from the computer in a single wire harness. It is further desirable to provide one assembly of solenoids having a single electrical connector assembly matable with the wire harness from the computer.
It is still further desirable to provide solenoids having two levels of responses to provide lesser or greater influence on the wheel braking systems.